Hematopoietic progenitor cells (HPCs) in bone marrow differentiate into both myeloid, and lymphoid cell lineages that disperse into different tissues throughout the body. Several studies indicate that HPCs can be infected by HIV-1 in vitro. Our preliminary data shows that mac-tropic R5 variants (adapted to use low CD4 on macrophages) infect umbilical cord blood HPCs more efficiently than non-mac-tropic R5 viruses and that infection via cell contact is particularly efficient. We also show that HPCs infected in vitro survive to differentiate and multiply into mature myeloid cells. Mature cells were then able to transfer virus to susceptible target cells. Our data indicates that infection of a single HPC would have the potential to produce large numbers of HIV+ mature cells. Such mature cells would be able to seed HIV into different tissues throughout the body. HIV sequences in bone marrow were reported to be closely related to those in brain. This result is consistent with HIV+ monocytes (derived from infected HPCs) seeding HIV into the brain, as reported to occur in late disease. Other limited studies have not confirmed this. However, bone marrow contains many cell types including mature macrophages and lymphoid cells, so that sampling of HIV may not detect virus from HPCs. Here, we will evaluate HIV sequences in purified HPCs and compare them with those in brain. In this proposal, our main goals are to investigate (1) whether HPCs from bone marrow of HIV+ subjects carry HIV and whether they can differentiate into myeloid or T-cell lineage cells that then transfer virus to new cells. And (2) whether HIV sequences present in HPCs are related to those in brain tissue, to support the presence of a bone marrow to brain dispersal route for HIV. We present three aims. Aim 1: Determine whether HIV-negative adult bone marrow HPCs can be infected in vitro and whether they retain the capacity to differentiate into myeloid and T-cell lineage cells. We will test if our data showing HIV infection and differentiation of umbilical cord blood HPCs applies to adult bone marrow HPCs Aim 2: Determine whether HPCs derived from HIV+ bone marrow are infected and can differentiate into infected myeloid and T-cell lineage cells. We will test differentiation of HIV+ bone marrow HPCs into both T- cell and myeloid lineage cells and establish whether mature cells carry infectious and transferable virus. Aim 3: Establish whether HIV sequences in HPCs are closely related to those in brain tissue. We will examine whether HIV sequences present in HIV+ bone marrow HPCs are closely related to viral quasispecies present in the brain of the same HIV+ patients with HIV-associated neurocognitive disorder (HAND). Our hypothesis is that HIV+ HPCs differentiate into mature myeloid and T-cells that disperse HIV through the body. If supported, our results will point to a major role for HPCs in HIV pathogenesis and formation viral reservoirs that has previously not been appreciated. This new information may lead to new strategies to identify HIV+ HPCs and derivatives that need to be eradicated if HIV is to be eliminated in vivo.